The invention relates to an electrical delay network comprising a chain of a plurality of (at least three) all-pass sections with constant amplitude response and mutually identical phase response as a function of the frequency. In addition to delay purposes, such a network is frequently also used as an electrical filter network, whose frequency response can be adjusted electrically by simple means. In this respect the trend in recent years is towards digitally controlled and operated filters, for example transversal filters, comprising a chain of clock-pulse controlled memory sections, whose tappings lead to a summing device via weighting factor adjusting means. Thus, this summing device adds the original signal to time-delayed signals, yielding an output signal whose amplitude and phase (i.e. the characteristic) can be adjusted electrically with the aid of the weighting factor adjusting means. Such filters can satisfactorily be realized with the aid of integrated circuits and are suitable for a wide field of applications owing to their adjustability, a desired adjustment being obtainable by means of a programmable read-only memory (ROM).
Nevertheless, such filters exhibit some drawbacks, which prohibit their general use in radio and television receivers. For example, not only a source of clock pulses is needed, but generally this source should also be capable of supplying substantial power, in order to shift the signal along the chain of memory sections with the required reliability and accuracy. Moreover, the clock frequency, which determines the maximum signal frequency that can be processed, cannot be varied without the risk of variable signal degredation.
The invention aims at a different approach, which is based on theories about so-called "ortho-normal" functions (see for example Y. W. Lee, Statistical Theory of Communication, 1960, chapter 19). As an example, one of the applications of an electrical network in accordance with the invention is that in which the chain of all-pass sections comprises tappings on which, owing to the use of weighting-factor adjusting means, adjustable signals are available, which by summation yield a transfer characteristic of the total network which is dependent on the adjustable weighting factors.
Such networks also add the signal to the delayed signals, while the weighting factor adjusting means provide separate signal processing without or substantially without any interaction owing to the orthonormality, so that again a wide range of frequency characteristics can be obtained. The invention is based on the recognition that such networks can also be realized in integrated circuit technology, provided that certain requirements are met.